1. Field of the Invention
The present invention relates to an ultrasonic cleaning apparatus and an ultrasonic cleaning method and, more specifically, to an ultrasonic cleaning apparatus and an ultrasonic cleaning method capable of cleaning at a high cleaning level without causing damage to objects to be cleaned.
2. Description of the Related Art
In an electronics industry, high cleaning levels are required in cleaning of glass substrates or substrates for silicon wafers. Examples of methods of cleaning objects to be cleaned such as the substrates as described above include a dip system in which a plurality of the substrates are immersed in a cleaning solution and a single wafer system in which the cleaning solution is injected onto the substrates and clean the same one by one. In recent years, the single wafer system which allows the cleaning at the high cleaning levels and is advantageous in terms of costs is predominantly employed. The dip system and the single wafer system are both included in a cleaning method in which ultrasonic oscillations are applied into the interior of cleaning solution to remove particulate contamination from the objects to be cleaned by oscillating actions thereof, and are put to practical use as the ultrasonic cleaning.
For example, in the ultrasonic cleaning of the dip system, the ultrasonic oscillations are applied toward the objects to be cleaned such as the substrates immersed in a container filled with the cleaning solution. Minute air bubbles (cavitation air bubbles) generated when the ultrasonic oscillations are propagated in liquid oscillate in accordance with positive and negative cycles of the ultrasonic oscillations, and remove the particulate contamination existing in the periphery thereof. However, when an amplitude of the ultrasonic oscillations increases, the air bubbles are broken in the positive cycle, and an impact wave generated at that time causes damage to the objects to be cleaned. In particular, in low-frequency ultrasonic waves not exceeding 100 kHz, the amplitudes of the ultrasonic oscillations are large, and hence the objects to be cleaned are subject to damage. Therefore, in the cleaning of the glass substrates or the silicon wafers in the electronics industry, high-frequency ultrasonic waves at 400 kHz or higher the amplitudes of the ultrasonic oscillations of which are small are employed.
JP-A-10-309548, JP-A-10-106998, International Publication WO00/21692, and JP-T-2005-512340 disclose a single wafer system ultrasonic cleaning. According to the ultrasonic cleaning disclosed in JP-A-10-309548, a cleaning solution is introduced into a head having an ultrasonic generator built therein, then the cleaning solution is supplied from an exit side of the head toward an object to be cleaned to be dropped into a cleaning solution tank, and the object to be cleaned is moved by being pulled perpendicularly upward from the cleaning solution tank. Surface acoustic waves or plate waves are excited in the object to be cleaned by the supply of the cleaning solution and application of the ultrasonic waves by the head, and the cleaning solution being in contact with the surface of the object to be cleaned is provided with a flowing force, and a current distribution of the cleaning solution presents a high flowing velocity in the vicinity of the surface. In this manner, the particulate contamination adhered to an upper surface of the object to be cleaned is separated by the surface acoustic waves or the plate waves.
According to the ultrasonic cleaning disclosed in JP-A-10-106998, ultrasonic oscillations are applied to a cleaning solution supplied to a container opened on top to push up the cleaning solution from a horizontal surface. In a state in which a lower surface of an object to be cleaned such as a substrate is in contact with the cleaning solution pushed upward from the horizontal surface, the object to be cleaned is moved in the horizontal direction. At the same time, the cleaning solution is supplied from an upper surface of the object to be cleaned. Part of the ultrasonic wave applied from the lower surface of the object to be cleaned penetrates through the object to be cleaned to the upper surface thereof, and acts on a particulate contamination adhered to the upper surface of the object to be cleaned, whereby a cleaning process is performed.
According to the ultrasonic cleaning disclosed in International Publication WO00/21692, a transducer having an irradiation surface area being comparable with an object to be cleaned immersed in a container filled with a cleaning solution is arranged in parallel thereto, and ultrasonic oscillations are applied toward a lower surface of the object to be cleaned. At this time, part of the ultrasonic oscillations penetrates through the object to be cleaned, and acts on a particulate contamination adhered to an upper surface of a substrate to be cleaned, whereby the both surfaces of the substrate to be cleaned are cleaned simultaneously. Accordingly, a period required for cleaning the object to be cleaned is reduced.
JP-T-2005-512340 discloses a processing apparatus illustrated in FIG. 16. As illustrated in FIG. 16, megasonic vibrating units 304 are arranged at both ends of an upper portion of a chamber 300 obliquely with respect to a liquid surface, and a substrate S is moved by substrate supporting notches 302 in the perpendicular direction. A cleaning solution is supplied from a bottom of the chamber 300 to cause the cleaning solution to drop from weirs at the upper portion of the chamber 300. The processing apparatus is configured to perform cleaning on the substrates by irradiating the substrates directly with energy E from the megasonic vibrating unit 304 while moving the substrates S in the perpendicular direction by the substrate supporting the notches 302.
However, in the dip-system ultrasonic cleaning of the related art in which a plurality of the substrates are cleaned simultaneously, the cleaning solution flows along the surfaces of the objects to be cleaned, but layers of several microns or smaller flowing little or never exist along interfaces. Therefore, there is a problem that the contamination adhered to the surfaces of the objects to be cleaned may not be removed sufficiently. Although application of the ultrasonic waves is effective as a device for destabilizing the layer of the cleaning solution in the vicinity of the surfaces of the objects to be cleaned, there is a problem that sufficient cleaning is not achieved with an ultrasonic wave on the order of 1 MHz the amplitude of the ultrasonic oscillations of which is small.
Furthermore, in a case of a semiconductor device progressed in miniaturization, occurrence of damage is reported even with the ultrasonic wave on the order of 1 MHz the amplitude of the ultrasonic oscillations of which is small. In all of the single wafer system cleaning methods of the related art disclosed in JP-A-10-309548, JP-A-10-106998, International Publication WO00/21692, and JP-T-2005-512340, the objects to be cleaned by themselves are oscillated by applying the ultrasonic waves directly to the back sides of the objects to be cleaned having no fine pattern, whereby ultrasonic energy is propagated to minute devices on the surfaces of the substrates. Therefore, the semiconductor device progressed in miniaturization may be subject to occurrence of damage by being influenced by the ultrasonic oscillations even with the ultrasonic wave on the order of 1 MHz the amplitude of the ultrasonic oscillations of which is small.